Interpretive Summary: Mastitis is the most economically important disease in the dairy industry. Recently the use of antibiotics to treat disease in veterinary medicine has come under scrutiny due to the concern of residues entering the human food chain. This paper demonstrates the ability of vitamin D to affect the severity of disease. Dairy cows were infected with a common mastitis pathogen and half the animals were treated with a natural form of vitamin D. Treated cows after 2 days of infection had an average of 10,000 fewer bacteria per milliliter of milk than the non-treated animals. The treated cow also had fewer clinical signs of a severe infection than the non-treated animals. The use of vitamin D to enhance the immune system may be a means to decrease the use of antibiotics in the food chain.

Technical Abstract:
Deficiency of serum levels of 25-hydroxyvitamin D3 has been correlated with increased risk of infectious diseases, such as tuberculosis and influenza. A plausible reason for this association is that expression of genes encoding important antimicrobial proteins depends on concentrations of 1,25-dihydroxyvitamin D3 produced by activated immune cells at sites of infection, and that synthesis of 1,25-dihydroxyvitamin D3 is dependent on the availability of 25-hydroxyvitamin D3. Thus, increasing the availability of 25(OH)D3 for immune cell synthesis of 1,25-dihydroxyvitamin D3 at sites of infection has been hypothesized to aid in clearance of the infection. This report details the treatment of an acute intramammary infection with infusion of 25-hydroxyvitamin D3 to the site of infection. Ten lactating cows were infected with Streptococcus uberis in one quarter of their mammary glands. Half of the animals were treated intramammary with 25-hydroxyvitamin D3. The 25-hydroxyvitamin D3 treated animal showed significantly lower bacterial counts in milk and showed reduced symptomatic affects of the mastitis. It is significant that treatment with 25-hydroxyvitamin D3 reduced the severity of an acute bacterial infection. This finding suggested a significant non-antibiotic complementary role for 25-hydroxyvitamin D3 in the treatment of infections in compartments naturally low in 25-hydroxyvitamin D3, such as the mammary gland and by extension, possibly upper respiratory tract infections.